chore: replace hard-coded obs values with constants throughout all the source code (#2037)

* chore: replace hard-coded OBS values with constants throughout all the source code

* chore(tests): replace hard-coded OBS values with constants throughout all the test code

tests/rl/test_actor_learner.py

@@ -24,6 +24,7 @@ from torch.multiprocessing import Event, Queue
 
 from lerobot.configs.train import TrainRLServerPipelineConfig
 from lerobot.policies.sac.configuration_sac import SACConfig
+from lerobot.utils.constants import OBS_STR
 from lerobot.utils.transition import Transition
 from tests.utils import require_package
 
@@ -33,12 +34,12 @@ def create_test_transitions(count: int = 3) -> list[Transition]:
     transitions = []
     for i in range(count):
         transition = Transition(
-            state={"observation": torch.randn(3, 64, 64), "state": torch.randn(10)},
+            state={OBS_STR: torch.randn(3, 64, 64), "state": torch.randn(10)},
             action=torch.randn(5),
             reward=torch.tensor(1.0 + i),
             done=torch.tensor(i == count - 1),  # Last transition is done
             truncated=torch.tensor(False),
-            next_state={"observation": torch.randn(3, 64, 64), "state": torch.randn(10)},
+            next_state={OBS_STR: torch.randn(3, 64, 64), "state": torch.randn(10)},
             complementary_info={"step": torch.tensor(i), "episode_id": i // 2},
         )
         transitions.append(transition)